Method for feeding back channel state information in wireless communication system and apparatus therefor

ABSTRACT

The present invention relates to a method for receiving a reference CSI configuration information and a following CSI configuration information which is configured to report a same RI (Rank Indicator) as the reference CSI configuration information, receiving a first CSI-RS (Channel State Information-Resource Signal) configuration information associated with the reference CSI configuration and a second CSI-RS configuration information associated with the following CSI configuration, number of CSI-RS antenna ports according to the second CSI-RS configuration information is same as number of CSI-RS antenna ports according to the first CSI-RS configuration information; and transmitting CSI determined based on at least one of the first CSI-RS configuration information and the second CSI-RS configuration information.

This application is a continuation of U.S. patent application Ser. No.14/383,220, filed Sep. 5, 2014, which application is a national stageentry of International Application No. PCT/KR2013/009418, filed Oct. 22,2013, which claims priority to U.S. Provisional Application No.61/717,628, filed Oct. 23, 2012, and 61/723,297, filed Nov. 6, 2012, allof which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to wireless communication systems, andmore particularly, to a method for feeding back channel stateinformation in a wireless communication system and an apparatustherefor.

BACKGROUND ART

As an example of a wireless communication system to which the presentinvention is applicable, a 3rd Generation Partnership Project Long TermEvolution (3GPP LTE) (hereinafter, referred to as ‘LTE’) communicationsystem is briefly described.

FIG. 1 is a view schematically illustrating the network architecture ofan E-UMTS as an exemplary wireless communication system. An EvolvedUniversal Mobile Telecommunications System (E-UMTS) is an advancedversion of a legacy Universal Mobile Telecommunications System (UMTS)and standardization thereof is currently underway in the 3GPP. E-UMTSmay be generally referred to as an LTE system. For details of thetechnical specifications of UMTS and E-UMTS, reference can respectivelybe made to Release 7 and Release 8 of “3rd Generation PartnershipProject; Technical Specification Group Radio Access Network”.

Referring to FIG. 1, the E-UMTS includes a User Equipment (UE), eNode Bs(eNBs), and an Access Gateway (AG) which is located at an end of anetwork (Evolved-Universal Terrestrial Radio Access Network ((E-UTRAN))and connected to an external network. The eNBs may simultaneouslytransmit multiple data streams for a broadcast service, a multicastservice, and/or a unicast service.

One or more cells may exist in one eNB. A cell is configured to use oneof bandwidths of 1.25, 2.5, 5, 10, 20 MHz to provide a downlink oruplink transport service to several UEs. Different cells may beconfigured to provide different bandwidths. The eNB controls datatransmission and reception for a plurality of UEs. The eNB transmitsdownlink scheduling information for downlink data to notify acorresponding UE of a data transmission time/frequency domain, coding,data size, and Hybrid Automatic Repeat and reQuest (HARQ)-relatedinformation. In addition, the eNB transmits uplink schedulinginformation for uplink data to inform a corresponding UE of availabletime/frequency domains, coding, data size, and HARQ-related information.An interface for transmitting user traffic or control traffic may beused between eNBs. A Core Network (CN) may include an AG and a networknode for user registration of the UE. The AG manages mobility of the UEon a Tracking Area (TA) basis, wherein one TA consists of a plurality ofcells.

Although radio communication technology has been developed up to LTEbased on Wideband Code Division Multiple Access (WCDMA), demands andexpectations of users and service providers have continued to increase.In addition, since other radio access technologies continue to bedeveloped, new technical evolution is required for futurecompetitiveness. Decrease of cost per bit, increase of serviceavailability, flexible use of a frequency band, simple structure andopen interface, and suitable power consumption by a UE are required.

To aid in efficient management of a wireless communication system of aneNB, a UE periodically and/or aperiodically reports state information ofa current channel to the eNB. Since the reported state information ofthe channel may include results calculated in consideration of varioussituations, a more efficient reporting method is needed.

DISCLOSURE Technical Problem

An object of the present invention devised to solve the problem lies inproviding a method for reporting channel state information in a wirelesscommunication system and an apparatus therefor.

It will be appreciated by persons skilled in the art that that thetechnical objects that can be achieved through the present invention arenot limited to what has been particularly described hereinabove andother technical objects of the present invention will be more clearlyunderstood from the following detailed description.

Technical Solution

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, in awireless access system supportive of multiple cells, a method ofreceiving a reference CSI configuration information and a following CSIconfiguration information which is configured to report a same RI (RankIndicator) as the reference CSI configuration information, receiving afirst CSI-RS (Channel State Information-Resource Signal) configurationinformation associated with the reference CSI configuration and a secondCSI-RS configuration information associated with the following CSIconfiguration, wherein number of CSI-RS antenna ports according to thesecond CSI-RS configuration information is same as number of CSI-RSantenna ports according to the first CSI-RS configuration information;and transmitting CSI determined based on at least one of the firstCSI-RS configuration information and the second CSI-RS configurationinformation.

According to one embodiment, the first CSI-RS configuration informationand the second CSI-RS configuration information are related to CSI-RS ofnon-zero transmission power respectively.

According to one embodiment, the first CSI-RS configuration informationand the second CSI-RS configuration information are transmitted throughRRC (Radio Resource Control) signaling respectively.

According to one embodiment, the reference CSI configuration informationand the following CSI configuration information are transmitted throughRRC (Radio Resource Control) signaling respectively.

According to one embodiment, the CSI includes at least one of RI, PMI(Precoding Matrix Indicator), and CQI (Channel Quality Indicator).

To further achieve these and other advantages and in accordance with thepurpose of the present invention, in a wireless access system supportiveof multiple cells, a method of receiving a reference CSI configurationinformation and a following CSI configuration information which isconfigured to report a same RI (Rank Indicator) as the reference CSIconfiguration information, receiving a first CSI-RS (Channel StateInformation-Resource Signal) configuration information associated withthe reference CSI configuration and a second CSI-RS configurationinformation associated with the following CSI configuration, whereinnumber of CSI-RS antenna ports according to the second CSI-RSconfiguration information is same as number of CSI-RS antenna portsaccording to the first CSI-RS configuration information, andtransmitting CSI determined based on at least one of the first CSI-RSconfiguration information and the second CSI-RS configurationinformation.

According to one embodiment, the first CSI-RS configuration informationand the second CSI-RS configuration information are related to CSI-RS ofnon-zero transmission power respectively.

According to one embodiment, the first CSI-RS configuration informationand the second CSI-RS configuration information are transmitted throughRRC (Radio Resource Control) signaling respectively.

According to one embodiment, the reference CSI configuration informationand the following CSI configuration information are transmitted throughRRC (Radio Resource Control) signaling respectively.

According to one embodiment, the CSI includes at least one of RI, PMI(Precoding Matrix Indicator), and CQI (Channel Quality Indicator).

To further achieve these and other advantages and in accordance with thepurpose of the present invention, in a wireless access system supportiveof multiple cells, a mobile station including a RF (Radio Frequency)module and a processor configured to: receive a reference CSIconfiguration information and a following CSI configuration informationwhich is configured to report a same RI (Rank Indicator) as thereference CSI configuration information, receive a first CSI-RS (ChannelState Information-Resource Signal) configuration information associatedwith the reference CSI configuration and a second CSI-RS configurationinformation associated with the following CSI configuration, whereinnumber of CSI-RS antenna ports according to the second CSI-RSconfiguration information is same as number of CSI-RS antenna portsaccording to the first CSI-RS configuration information, and transmitCSI determined based on at least one of the first CSI-RS configurationinformation and the second CSI-RS configuration information.

To further achieve these and other advantages and in accordance with thepurpose of the present invention, in a wireless access system supportiveof multiple cells, a base station including a RF (Radio Frequency)module and a processor configured to: receive a reference CSIconfiguration information and a following CSI configuration informationwhich is configured to report a same RI (Rank Indicator) as thereference CSI configuration information, receive a first CSI-RS (ChannelState Information-Resource Signal) configuration information associatedwith the reference CSI configuration and a second CSI-RS configurationinformation associated with the following CSI configuration, whereinnumber of CSI-RS antenna ports according to the second CSI-RSconfiguration information is same as number of CSI-RS antenna portsaccording to the first CSI-RS configuration information, and transmitCSI determined based on at least one of the first CSI-RS configurationinformation and the second CSI-RS configuration information.

Advantageous Effects

According to embodiments of the present invention, channel stateinformation can be effectively reported in a wireless communicationsystem.

It will be appreciated by persons skilled in the art that that theeffects that can be achieved through the present invention are notlimited to what has been particularly described hereinabove and otheradvantages of the present invention will be more clearly understood fromthe following detailed description.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 schematically illustrates the network architecture of an E-UMTSas an exemplary wireless communication system;

FIG. 2 illustrates structures of a control plane and a user plane of aradio interface protocol between a UE and an E-UTRAN based on the 3GPPradio access network specification;

FIG. 3 illustrates physical channels used in a 3GPP system and a generalsignal transmission method using the same;

FIG. 4 illustrates the structure of a radio frame used in an LTE system;

FIG. 5 illustrates the structure of a downlink radio frame used in anLTE system;

FIG. 6 illustrates the structure of an uplink subframe used in the LTEsystem;

FIG. 7 illustrates the configuration of a general MIMO communicationsystem;

FIGS. 8 to 11 illustrate periodic reporting of CSI;

FIGS. 12 and 13 illustrate periodic reporting processes of CSI when anon-hierarchical codebook is used;

FIG. 14 illustrates a periodic reporting process of CSI when ahierarchical codebook is used;

FIG. 15 illustrates an example of performing CoMP;

FIG. 16 illustrates a downlink CoMP operation;

FIG. 17 illustrates the case in which a type 5 report of a following CSIprocess and a type 5 report of a reference CSI process collide;

FIG. 18 is a diagram showing another embodiment of the case in which atype 5 report of a following CSI process and a type 5 report of areference CSI process collide;

FIG. 19 is a diagram showing an embodiment in which three CSI processescollide as an extension of FIG. 18; and

FIG. 20 is a diagram showing a BS and a UE which are applicable to thepresent invention.

BEST MODE

Hereinafter, the structures, operations, and other features of thepresent invention will be understood readily from the embodiments of thepresent invention, examples of which are described with reference to theaccompanying drawings. The embodiments which will be described below areexamples in which the technical features of the present invention areapplied to a 3GPP system.

Although the embodiments of the present invention will be describedbased on an LTE system and an LTE-Advanced (LTE-A) system, the LTEsystem and the LTE-A system are only exemplary and the embodiments ofthe present invention can be applied to all communication systemscorresponding to the aforementioned definition. In addition, althoughthe embodiments of the present invention will herein be described basedon Frequency Division Duplex (FDD) mode, the FDD mode is only exemplaryand the embodiments of the present invention can easily be modified andapplied to Half-FDD (H-FDD) mode or Time Division Duplex (TDD) mode.

FIG. 2 is a view illustrating structures of a control plane and a userplane of a radio interface protocol between a UE and an E-UTRAN based onthe 3GPP radio access network specification. The control plane refers toa path through which control messages used by a User Equipment (UE) anda network to manage a call are transmitted. The user plane refers to apath through which data generated in an application layer, e.g. voicedata or Internet packet data, is transmitted.

A physical layer of a first layer provides an information transferservice to an upper layer using a physical channel. The physical layeris connected to a Medium Access Control (MAC) layer of an upper layervia a transport channel. Data is transported between the MAC layer andthe physical layer via the transport channel. Data is also transportedbetween a physical layer of a transmitting side and a physical layer ofa receiving side via a physical channel. The physical channel uses timeand frequency as radio resources. Specifically, the physical channel ismodulated using an Orthogonal Frequency Division Multiple Access (OFDMA)scheme in downlink and is modulated using a Single-Carrier FrequencyDivision Multiple Access (SC-FDMA) scheme in uplink.

A MAC layer of a second layer provides a service to a Radio Link Control(RLC) layer of an upper layer via a logical channel. The RLC layer ofthe second layer supports reliable data transmission. The function ofthe RLC layer may be implemented by a functional block within the MAC. APacket Data Convergence Protocol (PDCP) layer of the second layerperforms a header compression function to reduce unnecessary controlinformation for efficient transmission of an Internet Protocol (IP)packet such as an IPv4 or IPv6 packet in a radio interface having arelatively narrow bandwidth.

A Radio Resource Control (RRC) layer located at the bottommost portionof a third layer is defined only in the control plane. The RRC layercontrols logical channels, transport channels, and physical channels inrelation to configuration, re-configuration, and release of radiobearers. The radio bearers refer to a service provided by the secondlayer to transmit data between the UE and the network. To this end, theRRC layer of the UE and the RRC layer of the network exchange RRCmessages. The UE is in an RRC connected mode if an RRC connection hasbeen established between the RRC layer of the radio network and the RRClayer of the UE. Otherwise, the UE is in an RRC idle mode. A Non-AccessStratum (NAS) layer located at an upper level of the RRC layer performsfunctions such as session management and mobility management.

One cell of an eNB is set to use one of bandwidths such as 1.25, 2.5, 5,10, 15, and 20 MHz to provide a downlink or uplink transmission serviceto a plurality of UEs. Different cells may be set to provide differentbandwidths.

Downlink transport channels for data transmission from a network to a UEinclude a Broadcast Channel (BCH) for transmitting system information, aPaging Channel (PCH) for transmitting paging messages, and a downlinkShared Channel (SCH) for transmitting user traffic or control messages.Traffic or control messages of a downlink multicast or broadcast servicemay be transmitted through the downlink SCH or may be transmittedthrough an additional downlink Multicast Channel (MCH). Meanwhile,uplink transport channels for data transmission from the UE to thenetwork include a Random Access Channel (RACH) for transmitting initialcontrol messages and an uplink SCH for transmitting user traffic orcontrol messages. Logical channels, which are located at an upper levelof the transport channels and are mapped to the transport channels,include a Broadcast Control Channel (BCCH), a Paging Control Channel(PCCH), a Common Control Channel (CCCH), a Multicast Control Channel(MCCH), and a Multicast Traffic Channel (MTCH).

FIG. 3 is a view illustrating physical channels used in a 3GPP systemand a general signal transmission method using the same.

A UE performs initial cell search such as establishment ofsynchronization with an eNB when power is turned on or the UE enters anew cell (step S301). The UE may receive a Primary SynchronizationChannel (P-SCH) and a Secondary Synchronization Channel (S-SCH) from theeNB, establish synchronization with the eNB, and acquire informationsuch as a cell identity (ID). Thereafter, the UE may receive a physicalbroadcast channel from the eNB to acquire broadcast information withinthe cell. Meanwhile, the UE may receive a Downlink Reference Signal (DLRS) in the initial cell search step to confirm a downlink channel state.

Upon completion of initial cell search, the UE may receive a PhysicalDownlink Control Channel (PDCCH) and a Physical Downlink Shared Channel(PDSCH) according to information carried on the PDCCH to acquire moredetailed system information (step S302).

Meanwhile, if the UE initially accesses the eNB or if radio resourcesfor signal transmission are not present, the UE may perform a randomaccess procedure (steps S303 to S306) with respect to the eNB. To thisend, the UE may transmit a specific sequence through a Physical RandomAccess Channel (PRACH) as a preamble (steps S303 and S305), and receivea response message to the preamble through the PDCCH and the PDSCHcorresponding thereto (steps S304 and S306). In the case of acontention-based RACH, a contention resolution procedure may beadditionally performed.

The UE which performs the above procedures may receive a PDCCH/PDSCH(step S307) and transmit a Physical Uplink Shared Channel(PUSCH)/Physical Uplink Control Channel (PUCCH) (step S308) according toa general uplink/downlink signal transmission procedure. Especially, theUE receives Downlink Control Information (DCI) through the PDCCH. TheDCI includes control information such as resource allocation informationfor the UE and has different formats according to use purpose.

Meanwhile, control information, transmitted by the UE to the eNB throughuplink or received by the UE from the eNB through downlink, includes adownlink/uplink ACKnowledgment/Negative ACKnowledgment (ACK/NACK)signal, a Channel Quality Indicator (CQI), a Precoding Matrix Index(PMI), a Rank Indicator (RI), and the like. In the case of the 3GPP LTEsystem, the UE may transmit control information such as CQI/PMI/RIthrough the PUSCH and/or the PUCCH.

FIG. 4 is a view illustrating the structure of a radio frame used in anLTE system.

Referring to FIG. 4, the radio frame has a length of 10 ms (327200T_(s)) and includes 10 equally-sized subframes. Each of the subframeshas a length of 1 ms and includes two slots. Each of the slots has alength of 0.5 ms (15360 T_(s)). In this case, T_(s) denotes samplingtime and is represented by T_(s)=1/(15 kHz×2048)=3.2552×10⁻⁸ (about 33ns). Each slot includes a plurality of OFDM symbols in a time domain andincludes a plurality of Resource Blocks (RBs) in a frequency domain. Inthe LTE system, one resource block includes 12 subcarriers×7 (or 6) OFDMsymbols. A Transmission Time Interval (TTI), which is a unit time fordata transmission, may be determined in units of one or more subframes.The above-described structure of the radio frame is purely exemplary andvarious modifications may be made in the number of subframes included ina radio frame, the number of slots included in a subframe, or the numberof OFDM symbols included in a slot.

FIG. 5 is a view illustrating control channels contained in a controlregion of one subframe in a downlink radio frame.

Referring to FIG. 5, one subframe includes 14 OFDM symbols. The first tothird ones of the 14 OFDM symbols may be used as a control region andthe remaining 13 to 11 OFDM symbols may be used as a data region,according to subframe configuration. In FIG. 5, R1 to R4 representreference signals (RSs) or pilot signals for antennas 0 to 3,respectively. The RSs are fixed to a predetermined pattern within thesubframe irrespective of the control region and the data region. Controlchannels are allocated to resources to which the RS is not allocated inthe control region. Traffic channels are allocated to resources, towhich the RS is not allocated, in the data region. The control channelsallocated to the control region include a Physical Control FormatIndicator Channel (PCFICH), a Physical Hybrid-ARQ Indicator Channel(PHICH), a Physical Downlink Control Channel (PDCCH), etc.

The PCFICH, physical control format indicator channel, informs a UE ofthe number of OFDM symbols used for the PDCCH per subframe. The PCFICHis located in the first OFDM symbol and is established prior to thePHICH and the PDCCH. The PCFICH is comprised of 4 Resource ElementGroups (REGs) and each of the REGs is distributed in the control regionbased on a cell ID. One REG includes 4 Resource Elements (REs). The REindicates a minimum physical resource defined as one subcarrier×one OFDMsymbol. The PCFICH value indicates values of 1 to 3 or values of 2 to 4depending on bandwidth and is modulated by Quadrature Phase Shift Keying(QPSK).

The PHICH, physical Hybrid-ARQ indicator channel, is used to transmit aHARQ ACK/NACK signal for uplink transmission. That is, the PHICHindicates a channel through which downlink ACK/NACK information foruplink HARQ is transmitted. The PHICH includes one REG and iscell-specifically scrambled. The ACK/NACK signal is indicated by 1 bitand is modulated by Binary Phase Shift Keying (BPSK). The modulatedACK/NACK signal is spread by a Spreading Factor (SF)=2 or 4. A pluralityof PHICHs mapped to the same resource constitutes a PHICH group. Thenumber of PHICHs multiplexed to the PHICH group is determined dependingon the number of SFs. The PHICH (group) is repeated three times toobtain diversity gain in a frequency domain and/or a time domain.

The PDCCH, physical downlink control channel, is allocated to the firstn OFDM symbols of a subframe. In this case, n is an integer greater than1 and is indicated by the PCFICH. The PDCCH is comprised of one or moreControl Channel Elements (CCEs). The PDCCH informs each UE or UE groupof information associated with resource allocation of a Paging Channel(PCH) and a Downlink-Shared Channel (DL-SCH), uplink scheduling grant,Hybrid Automatic Repeat Request (HARQ) information, etc. Therefore, aneNB and a UE transmit and receive data other than specific controlinformation or specific service data through the PDSCH.

Information indicating to which UE or UEs PDSCH data is to betransmitted, information indicating how UEs are to receive PDSCH data,and information indicating how UEs are to perform decoding are containedin the PDCCH. For example, it is assumed that a specific PDCCH isCRC-masked with a Radio Network Temporary Identity (RNTI) “A” andinformation about data, that is transmitted using radio resources “B”(e.g., frequency location) and transport format information “C” (e.g.,transmission block size, modulation scheme, coding information, etc.),is transmitted through a specific subframe. In this case, a UE locatedin a cell monitors the PDCCH using its own RNTI information. If one ormore UEs having the RNTI ‘A’ are present, the UEs receive the PDCCH andreceive the PDSCH indicated by ‘B’ and ‘C’ through the received PDCCHinformation.

FIG. 6 illustrates the structure of an uplink subframe used in the LTEsystem.

Referring to FIG. 6, an uplink subframe is divided into a region towhich a PUCCH is allocated to transmit control information and a regionto which a PUSCH is allocated to transmit user data. The PUSCH isallocated to the middle of the subframe, whereas the PUCCH is allocatedto both ends of a data region in the frequency domain. The controlinformation transmitted on the PUCCH includes an ACK/NACK, a CQIrepresenting a downlink channel state, an RI for Multiple Input andMultiple Output (MIMO), a Scheduling Request (SR) indicating a requestfor allocation of uplink resources, etc. A PUCCH of a UE occupies one RBin a different frequency in each slot of a subframe. That is, two RBsallocated to the PUCCH frequency-hop over the slot boundary.Particularly, FIG. 6 illustrates an example in which PUCCHs for m=0,m=1, m=2, and m=3 are allocated to a subframe.

MIMO System

Hereinafter, a MIMO system will be described. MIMO refers to a method ofusing multiple transmission antennas and multiple reception antennas toimprove data transmission/reception efficiency. Namely, a plurality ofantennas is used at a transmitting end or a receiving end of a wirelesscommunication system so that capacity can be increased and performancecan be improved. MIMO may also be referred to as ‘multi-antenna’ in thisdisclosure.

MIMO technology does not depend on a single antenna path in order toreceive a whole message. Instead, MIMO technology collects datafragments received via several antennas, merges the data fragments, andforms complete data. The use of MIMO technology can increase systemcoverage while improving data transfer rate within a cell area of aspecific size or guaranteeing a specific data transfer rate. MIMOtechnology can be widely used in mobile communication terminals andrelay nodes. MIMO technology can overcome the limitations of therestricted amount of transmission data of single antenna based mobilecommunication systems.

The configuration of a general MIMO communication system is shown inFIG. 7. A transmitting end is equipped with N_(T) transmission (Tx)antennas and a receiving end is equipped with N_(R) reception (Rx)antennas. If a plurality of antennas is used both at the transmittingend and at the receiving end, theoretical channel transmission capacityincreases unlike the case where only either the transmitting end or thereceiving end uses a plurality of antennas. Increase in channeltransmission capacity is proportional to the number of antennas, therebyimproving transfer rate and frequency efficiency. If a maximum transferrate using a signal antenna is R_(o), a transfer rate using multipleantennas can be theoretically increased by the product of the maximumtransfer rate R_(o) by a rate increment R_(i). The rate increment R_(i)is represented by the following equation 1 where R_(i) is the smaller ofN_(T) and N_(R).R _(i)=min(N _(T) ,N _(R))  [Equation 1]

For example, in a MIMO communication system using four Tx antennas andfour Rx antennas, it is possible to theoretically acquire a transferrate four times that of a single antenna system. After theoreticalincrease in the capacity of the MIMO system was first demonstrated inthe mid-1990s, various techniques for substantially improving datatransfer rate have been under development. Several of these techniqueshave already been incorporated into a variety of wireless communicationstandards including, for example, 3^(rd) generation mobile communicationand next-generation wireless local area networks.

Active research up to now related to MIMO technology has focused upon anumber of different aspects, including research into information theoryrelated to MIMO communication capacity calculation in various channelenvironments and in multiple access environments, research into wirelesschannel measurement and model derivation of MIMO systems, and researchinto space-time signal processing technologies for improvingtransmission reliability and transfer rate.

To describe a communication method in a MIMO system in detail, amathematical model thereof is given below. As shown in FIG. 7, it isassumed that N_(T) Tx antennas and N_(R) Rx antennas are present. In thecase of a transmission signal, a maximum number of transmittable piecesof information is N_(T) under the condition that N_(T) Tx antennas areused, so that transmission information can be represented by a vectorrepresented by the following equation 2:s=[s₁ ,s ₂ , . . . ,s _(N) _(T) ]^(T)  [Equation 2]

Meanwhile, individual transmission information pieces s₁, s₂, . . . ,s_(N) _(T) may have different transmission powers. In this case, if theindividual transmission powers are denoted by P₁, P₂, . . . , P_(N) _(T), transmission information having adjusted transmission powers can berepresented by a vector shown in the following equation 3:ŝ=[ŝ₁ ,ŝ ₂ , . . . ,ŝ _(N) _(T) ]^(T)=[P₁ s ₁ ,P ₂ s ₂ , . . . ,P _(N)_(T) s _(N) _(T) ]^(T)  [Equation 3]

The transmission power-controlled transmission information vector ŝ maybe expressed as follows, using a diagonal matrix P of a transmissionpower:

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & \; & \; & 0 \\\; & {\; P_{2}} & \; & \; \\\; & \; & \ddots & \; \\0 & \; & \; & P_{N_{T}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\\vdots \\G_{N_{T}}\end{bmatrix}} = {Ps}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

*78

N_(T) transmission signals x₁, x₂, . . . , x_(N) _(T) to be actuallytransmitted may be configured by multiplying the transmissionpower-controlled information vector ŝ by a weight matrix W. In thiscase, the weight matrix is adapted to properly distribute transmissioninformation to individual antennas according to transmission channelsituations. The transmission signals x₁, x₂, . . . , x_(N) _(T) can berepresented by the following Equation 5 using a vector X. In Equation 5,W_(ij) is a weight between the i-th Tx antenna and the j-th informationand W is a weight matrix, which may also be referred to as a precodingmatrix.

$\begin{matrix}{x = {\quad{\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{i} \\\vdots \\x_{N_{T}}\end{bmatrix} = {{\begin{bmatrix}w_{11} & w_{12} & \ldots & w_{1N_{T}} \\w_{21} & w_{22} & \ldots & w_{2N_{T}} \\\vdots & \; & \ddots & \; \\w_{i\; 1} & w_{i\; 2} & \ldots & w_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\w_{N_{T}1} & w_{N_{T}2} & \ldots & w_{N_{T}N_{T}}\end{bmatrix}\begin{bmatrix}{\hat{s}}_{1} \\{\hat{s}}_{2} \\\vdots \\{\hat{s}}_{j} \\\vdots \\{\hat{s}}_{N_{T}}\end{bmatrix}} = {{W\hat{s}} = {WPs}}}}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

Generally, the physical meaning of a rank of a channel matrix may be amaximum number of different pieces of information that can betransmitted in a given channel. Accordingly, since the rank of thechannel matrix is defined as the smaller of the number of rows orcolumns, which are independent of each other, the rank of the matrix isnot greater than the number of rows or columns. A rank of a channelmatrix H, rank(H), is restricted as follows.rank(H)≦min(N _(T) ,N _(R))  [Equation 6]

Each unit of different information transmitted using MIMO technology isdefined as a ‘transmission stream’ or simply ‘stream’. The ‘stream’ maybe referred to as a ‘layer’. The number of transmission streams is notgreater than a rank of a channel which is a maximum number of differentpieces of transmittable information. Accordingly, the channel matrix Hmay be indicted by the following Equation 7:# of streams≦rank(H)≦min(N _(T) ,N _(R))  [Equation 7]

where ‘# of streams’ denotes the number of streams. It should be notedthat one stream may be transmitted through one or more antennas.

There may be various methods of allowing one or more streams tocorrespond to multiple antennas. These methods may be described asfollows according to types of MIMO technology. The case where one streamis transmitted via multiple antennas may be called spatial diversity,and the case where multiple streams are transmitted via multipleantennas may be called spatial multiplexing. It is also possible toconfigure a hybrid of spatial diversity and spatial multiplexing.

CSI Feedback

Now, a description of a Channel State Information (CSI) report is given.In the current LTE standard, a MIMO transmission scheme is categorizedinto open-loop MIMO operated without CSI and closed-loop MIMO operatedbased on CSI. Especially, according to the closed-loop MIMO system, eachof the eNB and the UE may be able to perform beamforming based on CSI toobtain a multiplexing gain of MIMO antennas. To obtain CSI from the UE,the eNB allocates a PUCCH or a PUSCH to command the UE to feedback CSIfor a downlink signal.

CSI is divided into three types of information: a Rank Indicator (RI), aPrecoding Matrix Index (PMI), and a Channel Quality Indicator (CQI).First, RI is information on a channel rank as described above andindicates the number of streams that can be received via the sametime-frequency resource. Since RI is determined by long-term fading of achannel, it may be generally fed back at a cycle longer than that of PMIor CQI.

Second, PMI is a value reflecting a spatial characteristic of a channeland indicates a precoding matrix index of the eNB preferred by the UEbased on a metric of Signal-to-Interference plus Noise Ratio (SINR).Lastly, CQI is information indicating the strength of a channel andindicates a reception SINR obtainable when the eNB uses PMI.

In an evolved communication system such as an LTE-A system, multi-userdiversity using Multi-User MIMO (MU-MIMO) is additionally obtained.Since interference between UEs multiplexed in an antenna domain existsin the MU-MIMO scheme, CSI accuracy may greatly affect not onlyinterference of a UE that has reported CSI but also interference ofother multiplexed UEs. Hence, in order to correctly perform MU-MIMOoperation, it is necessary to report CSI having accuracy higher thanthat of a Single User-MIMO (SU-MIMO) scheme.

Accordingly, LTE-A standard has determined that a final PMI should beseparately designed into W1, which a long-term and/or wideband PMI, andW2, which is a short-term and/or subband PMI.

An example of a hierarchical codebook transform scheme configuring onefinal PMI from among W1 and W2 may use a long-term covariance matrix ofa channel as indicated in Equation 8:W=norm(W1W2)  [Equation 8]

In Equation 8, W2 of a short-term PMI indicates a codeword of a codebookconfigured to reflect short-term channel information, W denotes acodeword of a final codebook, and norm(A) indicates a matrix in which anorm of each column of a matrix A is normalized to 1.

The detailed configurations of W1 and W2 are shown in Equation 9:

$\begin{matrix}{{{{W\; 1(i)} = \begin{bmatrix}X_{i} & 0 \\0 & X_{i}\end{bmatrix}},{{{where}\mspace{14mu} X_{i}\mspace{14mu}{is}\mspace{14mu}{{Nt}/2}\mspace{14mu}{by}\mspace{14mu} M\mspace{14mu}{{matrix}.W}\; 2(j)} = {\begin{bmatrix}e_{M}^{k} & e_{M}^{l} & e_{M}^{m} \\{\alpha_{j}e_{M}^{k}} & {\beta_{j}e_{M}^{l}} & {{\ldots\gamma}_{j}e_{M}^{m}}\end{bmatrix}\left( {{{if}\mspace{14mu}{rank}} = r} \right)}},{where}}{{1 \leq k},l,{m \leq M}}{and}{k,l,{m\mspace{14mu}{are}\mspace{14mu}{{integer}.}}}} & \left\lbrack {{Equation}\mspace{14mu} 9} \right\rbrack\end{matrix}$

where Nt is the number of Tx antennas, M is the number of columns of amatrix Xi, indicating that the matrix Xi includes a total of M candidatecolumn vectors. eMk, eMl, and eMm denote k-th, l-th, and m-th columnvectors of the matrix Xi in which only k-th, l-th, and m-th elementsamong M elements are 0 and the other elements are 0, respectively.α_(j), β_(j), and γ_(j) are complex values each having a unit norm andindicate that, when the k-th, l-th, and m-th column vectors of thematrix Xi are selected, phase rotation is applied to the column vectors.At this time, i is an integer greater than 0, denoting a PMI indexindicating W1 and j is an integer greater than 0, denoting a PMI indexindicating W2.

In Equation 9, the codebook configurations are designed to reflectchannel correlation properties generated when cross polarized antennasare used and when a space between antennas is dense, for example, when adistance between adjacent antennas is less than a half of signalwavelength. The cross polarized antennas may be categorized into ahorizontal antenna group and a vertical antenna group. Each antennagroup has the characteristic of a Uniform Linear Array (ULA) antenna andthe two groups are co-located.

Accordingly, a correlation between antennas of each group hascharacteristics of the same linear phase increment and a correlationbetween antenna groups has characteristics of phase rotation.Consequently, since a codebook is a value obtained by quantizing achannel, it is necessary to design a codebook such that characteristicsof a channel are reflected. For convenience of description, a rank-1codeword generated by the aforementioned configurations is shown asfollows:

$\begin{matrix}{{W\; 1(i)*W\; 2(j)} = \begin{bmatrix}{X_{i}(k)} \\{\alpha_{j}{X_{i}(k)}}\end{bmatrix}} & \left\lbrack {{Equation}\mspace{14mu} 10} \right\rbrack\end{matrix}$

In Equation 10, a codeword is expressed as a vector of N_(T)×1 (whereN_(T) is the number of Tx antennas) and is structured with an uppervector X_(i)(k) and a lower vector α_(j)X_(i)(k) which show correlationcharacteristics of a horizontal antenna group and a vertical antennagroup, respectively. X_(i)(k) is preferably expressed as a vector havingthe characteristics of linear phase increment by reflecting thecharacteristics of a correlation between antennas of each antenna groupand may be a DFT matrix as a representative example.

As described above, CSI in the LTE system includes, but is not limitedto, CQI, PMI, and RI. According to transmission mode of each UE, all orsome of the CQI, PMI, and RI is transmitted. Periodic transmission ofCSI is referred to as periodic reporting and transmission of CSI at therequest of an eNB is referred to as aperiodic reporting. In aperiodicreporting, a request bit included in uplink scheduling informationtransmitted by the eNB is transmitted to the UE. Then, the UE transmitsCSI considering transmission mode thereof to the eNB through an uplinkdata channel (PUSCH). In periodic reporting, a period of CSI and anoffset at the period are signaled in the unit of subframes by asemi-static scheme through a higher-layer signal per UE. The UEtransmits CSI considering transmission mode to the eNB through an uplinkcontrol channel (PUCCH). If there is uplink data in a subframe in whichCSI is transmitted, the CSI is transmitted through an uplink datachannel (PUSCH) together with the uplink data. The eNB transmitstransmission timing information suitable for each UE to the UE inconsideration of a channel state of each UE and a UE distributedsituation in a cell. The transmission timing information includes aperiod and an offset necessary for transmitting CSI and may betransmitted to each UE through an RRC message.

FIGS. 8 to 11 illustrate periodic reporting of CSI in an LTE system.

Referring to FIG. 8, there are four CQI reporting modes in the LTEsystem. Specifically, the CQI reporting modes may be divided into modesin a WideBand (WB) CQI and modes in a SubBand (SB) CQI according to CQIfeedback type. The CQI reporting mode may also be divided into modes ina No PMI and modes in a single PMI depending on whether a PMI istransmitted or not. Each UE is informed of information comprised of aperiod and an offset through RRC signaling in order to periodicallyreport CQI.

FIG. 9 illustrates an example of transmitting CSI when a UE receivesinformation indicating {a period ‘5’ and an offset ‘1’} throughsignaling. Referring to FIG. 9, upon receiving the informationindicating the period ‘5’ and offset ‘1’, the UE transmits CSI in theunit of 5 subframes with an offset of one subframe in ascending order ofa subframe index counted from 0 starting from the first subframe.Although the CSI is transmitted basically through a PUCCH, if a PUSCHfor data transmission is present at the same transmission time point,the CSI is transmitted through the PUSCH together with data. Thesubframe index is given as a combination of a system frame number (or aradio frame index) n_(f) and a slot index n_(s) (0 to 19). Since onesubframe includes two slots, the subframe index may be defined as10×n_(f)+floor(n_(s)/2) wherein floor( ) indicates the floor function.

CQI transmission types include a type of transmitting a WB CQI only anda type of transmitting both a WB CQI and an SB CQI. In the type oftransmitting a WB CQI only, CQI information for all bands is transmittedin subframes corresponding to every CQI transmission period. Meanwhile,in the case in which PMI information should also be transmittedaccording to the PMI feedback type as illustrated in FIG. 8, the PMIinformation is transmitted together with the CQI information. In thetype of transmitting both a WB CQI and an SB CQI, the WB CQI and SB CQIare alternately transmitted.

FIG. 10 illustrates a system in which a system bandwidth consists of 16RBs. It is assumed that the system bandwidth includes two BandwidthParts (BPs) BP0 and BP1 each consisting of two SubBands (SBs) SB0 andSB1 and each SB includes 4 RBs. The above assumption is exemplary andthe number of BPs and the size of each SB may vary with the size of thesystem bandwidth. The number of SBs constituting each BP may differaccording to the number of RBs, the number of BPs, and the size of eachSB.

In the CQI transmission type of transmitting both a WB CQI and an SBCQI, the WB CQI is transmitted in the first CQI transmission subframeand an SB CQI of the better SB state of SB0 and SB1 in BP0 istransmitted in the next CQI transmission subframe together with and anindex of the corresponding SB (e.g. Subband Selection Indicator (SSI).Thereafter, an SB CQI of the better SB state of SB0 and SB1 in BP1 andan index of the corresponding SB are transmitted in the next CQItransmission subframe. Thus, CQI of each BP is sequentially transmittedafter transmission of the WB CQI. The CQI of each BP may be sequentiallytransmitted once to four times during the interval between transmissionintervals of two WB CQIs. For example, if the CQI of each BP istransmitted once during the time interval between two WB CQIs, CQIs maybe transmitted in the order of WB CQI

BP0 CQI

BP1 CQI

WB CQI. If the CQI of each BP is transmitted four times during the timeinterval between two WB CQIs, CQIs may be transmitted in the order of WBCQI

BP0 CQI

BP1 CQI

BP0 CQI

BP1 CQI

BP0 CQI

BP1 CQI

BP0 CQI

BP1 CQI

WB CQI. Information as to how many times each BP CQI is transmitted issignaled by a higher layer (RRC layer).

FIG. 11(a) illustrates an example of transmitting both a WB CQI and anSB CQI when a UE receives information indicating {period ‘5’ and offset‘1’} through signaling. Referring to FIG. 11(a), a CQI may betransmitted only in subframes corresponding to the signaled period andoffset regardless of type. FIG. 11(b) illustrates an example oftransmitting an RI in addition to the example shown in FIG. 11(a). TheRI may be signaled as a combination of a multiple of a WB CQItransmission period and an offset at the transmission period from ahigher layer (e.g. RRC layer). The offset of the RI is signaled using avalue relative to the offset of a CQI. For example, if the offset of theCQI is ‘1’ and the offset of the RI is ‘0’, the RI has the same offsetas the CQI. The offset value of the RI is defined as 0 or a negativenumber. More specifically, it is assumed in FIG. 11(b) that, in anenvironment identical to that of FIG. 11(a), an RI transmission periodis a multiple of 1 of the WB CQI transmission period and the RI offsetis ‘−1’. Since the RS transmission period is a multiple of 1 of the WBCQI transmission period, the RS transmission period and the WB CQItransmission period are substantially the same. Since the offset of theRI is ‘−1’, the RI is transmitted based upon the value ‘−1’ (i.e.subframe index 0) relative to the offset ‘1’ of the CQI in FIG. 11(a).If the offset of the RI is ‘0’, the transmission subframes of the WB CQIand RI overlap. In this case, the WB CQI is dropped and the RI istransmitted.

FIG. 12 illustrates CSI feedback in the case of Mode 1-1 of FIG. 8.

Referring to FIG. 12, CSI feedback is comprised of two types of reportcontent, i.e. transmission of Report 1 and transmission of Report 2.More specifically, an RI is transmitted through Report 1 and a WB PMIand a WB CQI are transmitted through Report 2. Report 2 is transmittedin subframe indexes satisfying(10*n_(f)+floor(ns/2)−N_(offset,CQI))mod(N_(pd))=0. N_(offset,CQI)indicates an offset for PMI/CQI transmission shown in FIG. 9. In FIG.12, N_(offset,CQI)=1. N_(pd) illustrates an interval of subframesbetween contiguous Reports 2 and the case of N_(pd)=2 is illustrated inFIG. 12. Report 1 is transmitted in subframe indexes satisfying(10*n_(f)+floor(n_(s)/2)−N_(offset,CQI)−N_(offset,RI))mod(M_(RI)*N_(pd))=0.M_(RI) is determined by higher layer signaling. N_(offset,RI) denotes arelative offset value for RI transmission shown in FIG. 11. The case inwhich M_(RI)=4 and N_(offset,RI)=−1 is illustrated in FIG. 12.

FIG. 13 illustrates CSI feedback in the case of Mode 2-1 of FIG. 8.

Referring to FIG. 13, CSI feedback is comprised of three types of reportcontent, i.e. transmission of Report 1, transmission of Report 2, andtransmission of Report 3. More specifically, an RI is transmittedthrough Report 1, a WB PMI and a WB CQI are transmitted through Report2, and an SB CQI and an L-bit Subband Selection Indicator (SSI) aretransmitted through Report 3. Report 2 or Report 3 is transmitted insubframe indexes satisfying(10*n_(f)+floor(n_(s)/2)−N_(offset,CQI))mod(N_(pd))=0. Especially,Report 2 is transmitted in subframe indexes satisfying(10*n_(f)+floor(n_(s)/2)−N_(offset,CQI))mod(H*N_(pd))=0. Accordingly,Report 2 is transmitted at an interval of H*N_(pd) and subframes betweencontiguous Reports are filled with transmission of Report 3. At thistime, H equals to J*K+1 wherein J is the number of BPs. K is a valueindicating how many full cycles will be consecutively performed, whereinthe full cycle is a cycle during which a process for selectivelytransmitting a subband once per different BP over all BPs. K isdetermined by higher layer signaling. The case in which N_(pd)=2, J=3,and K=1 is illustrated in FIG. 13. Report 1 is transmitted in subframeindexes satisfying(10*n_(f)+floor(n_(s)/2)−N_(offset,CQI)−N_(offset,RI))mod(M_(RI)*(J*K+1)*N_(pd))=0.The case in which M_(RI)=2 and N_(offset,RI)=−1 is illustrated in FIG.13.

FIG. 14 illustrates periodic reporting of CSI which is being discussedin LTE-A. If an eNB includes 8 Tx antennas in Mode 2-1, then a 1-bitindicator, i.e. a Precoder Type Indication (PTI) parameter, isconfigured and periodic reporting modes classified into two typesaccording to the PTI value are considered. In FIG. 14, W1 and W2illustrate hierarchical codebooks described with reference to Equations8 and 9. If both W1 and W2 are determined, a completed type of aprecoding matrix W is determined by combining W1 and W2.

Referring to FIG. 14, in the case of periodic reporting, differentcontents corresponding to Report 1, Report 2, and Report 3 are reportedaccording to different repetition periods. An RI and a 1-bit PTI valueare reported through Report 1. A WB W1 (when PTI=0) or a WB W2 and a WBCQI (when PTI=1) are reported through Report 2. A WB W2 and a WB CQI(when PTI=0) or an SB W2 and an SB CQI (when PTI=1) are reported throughReport 3.

Report 2 and Report 3 are transmitted in subframes (for convenience,referred to as a first subframe set) having subframe indexes satisfying(10*n_(f)+floor(n_(s)/2)−N_(offset,CQI))mod(N_(C))=0 whereinN_(offset,CQI) is an offset value for PMI/CQI transmission shown in FIG.9 and N_(c) denotes a subframe interval between contiguous Reports 2 orReports 3. The case in which N_(offset,CQI)=1 and N_(c)=2 is illustratedin FIG. 14. The first subframe set is comprised of subframes havingodd-numbered indexes. n_(f) denotes a system frame number (or radioframe index) and n_(s) denotes a slot index in a radio frame. floor( )indicates the floor function and ‘A mod B’ indicates a remainderobtained by dividing A by B.

Report 2 is located in some subframes in the first subframe set andReport 3 is located in the other subframes. More specifically, Report 2is located in subframes having subframe indexes satisfying(10*n_(f)+floor(n_(s)/2)−N_(offset,CQI))mod(H*N_(c))=0. Accordingly,Report 2 is transmitted at an interval of H*N_(c) and one or more firstsubframes between contiguous Reports 2 are filled with transmission ofReport 3. If PTI=0, then H=M and M is determined by higher layersignaling. The case in which M=2 is illustrated in FIG. 14. If PTI=1,then H=J*K+1, K is determined by higher layer signaling, and J is thenumber of BPs. In FIG. 14, J=3 and K=1.

Report 1 is transmitted in subframes having subframe indexes satisfying(10*n_(f)+floor(n_(s)/2)−N_(offset,CQI)−N_(offset,RI))mod(M_(RI)*(J*K+1)*N_(c))=0wherein M_(RI) is determined by higher layer signaling. N_(offset,RI)indicates a relative offset value for an RI. In FIG. 14, M_(RI)=2 andN_(offset,RI)=−1. The transmission time points of Report 1 and Report 2do not overlap because N_(offset,RI)=−1. When a UE calculates RI, W1,and W2, they are associated with each other. For example, W1 and W2 arecalculated depending on RI and W2 is calculated depending on W1. A BSmay be aware of a final W from W1 and W2 when both Report 2 and Report 3are reported after Report 1 is reported.

CSI Feedback of CoMP

Hereinafter, Cooperative Multipoint (CoMP) transmission/reception willbe described.

In a system after LTE-A, a scheme for raising system performance byenabling cooperation between a plurality of cells is attempted. Such ascheme is called CoMP transmission/reception. CoMP refers to a scheme inwhich two or more eNBs, access points, or cells cooperativelycommunicate with a UE for smooth communication between a specific UE andan eNB, an access point, or a cell. In the present invention, eNB,access point, and cell may be used interchangeably.

In general, in a multi-cell environment in which a frequency reusefactor is 1, the performance of the UE located at a cell edge andaverage sector throughput may be reduced due to Inter-Cell Interference(ICI). In order to reduce ICI, a legacy LTE system uses a method ofenabling the UE located at a cell edge to have appropriate throughputand performance using a simple passive scheme such as FractionalFrequency Reuse (FFR) through UE-specific power control in anenvironment restricted by interference. However, it is desirable thatICI be reduced or the UE reuse ICI as a desired signal, rather thandecreasing the use of frequency resources per cell. In order toaccomplish the above purpose, a CoMP transmission scheme may beemployed.

FIG. 15 illustrates an example of performing CoMP. Referring to FIG. 15,a radio communication system includes a plurality of eNBs eNB1, eNB2,and eNB3 that perform CoMP and a UE. The plural eNBs eNB1, eNB2, andeNB3 for performing CoMP may efficiently transmit data to the UE throughcooperation.

A CoMP transmission scheme may be divided into CoMP-Joint Processing(CoMP-JP) which is a cooperative MIMO type of JP through data sharingand CoMP-Coordinated Scheduling/Coordinated Beamforming (CoMP-CS/CB).

In the case a CoMP-JP scheme in downlink, a UE may simultaneouslyreceive data from a plurality of eNB implementing the CoMP transmissionscheme and may improve reception performance by combining signalsreceived from the respective eNBs (Joint Transmission (JT)). Inaddition, a method in which one of a plurality of eNBs performing theCoMP transmission scheme transmits data to the UE at a specific timepoint may be considered (Dynamic Point Selection (DPS). In a CoMP-CS/CBscheme in downlink, the UE may instantaneously receive data through oneeNB, i.e. a serving eNB by beamforming.

If the CoMP-JP scheme is applied in uplink, a plurality of eNBs maysimultaneously receive a PUSCH signal from the UE (Joint Reception(JR)). In the case of CoMP-CS/CB in uplink, only one eNB may receive aPUSCH signal. Cooperative cells (or eNBs) may determine to use theCoMP-CS/CB scheme.

A UE using the CoMP transmission scheme, i.e. a CoMP UE, may feed backchannel information feedback (hereinafter, CSI feedback) to a pluralityof eNBs performing the CoMP transmission scheme. A network scheduler mayselect a proper CoMP transmission scheme capable of raising atransmission rate among the CoMP-JP, CoMP-CS/CB, and DPS schemes basedon CSI feedback. To this end, a periodic feedback transmission schemeusing a PUCCH may be used as a method in which the UE configures CSIfeedback in a plurality of eNBs performing the CoMP transmission scheme.In this case, feedback configurations for the eNBs may be independent ofone another. Accordingly, in the disclosure according to an embodimentof the present invention, an operation of feeding back CSI with such anindependent feedback configuration is referred to as a CSI process. Oneor more CSI processes may be performed in one serving cell.

FIG. 16 illustrates a downlink CoMP operation.

In FIG. 16, a UE is positioned between an eNB1 and an eNB2 and the twoeNBs, i.e. eNB1 and eNB2, perform a proper CoMP operation such as JT,DCS, or CS/CB to solve a problem of interference to the UE. To aid inthe CoMP operation of the eNBs, the UE performs proper CSI feedback.Information transmitted through CSI feedback includes PMI and CQI ofeach eNB and may additionally include channel information between thetwo eNBs (e.g. phase offset information between two eNB channels) forJT.

In FIG. 16, although the UE transmits a CSI feedback signal to the eNB1which is a serving cell thereof, the UE may transmit the CSI feedbacksignal to the eNB2 or the two eNBs, according to situation. In addition,in FIG. 16, while the eNBs are described as a basic unit participatingin CoMP, the present invention may be applied to CoMP betweenTransmission Points (TPs) controlled by a single eNB.

That is, for CoMP scheduling in a network, the UE should feed back notonly downlink CSI of a serving eNB/TP but also downlink CSI of aneighboring eNB/TP. To this end, the UE feeds back a plurality of CSIprocesses reflecting various interference environments of eNBs/TPs fordata transmission.

Accordingly, an Interference Measurement Resource (IMR) is used tomeasure interference during CoMP CSI calculation in an LTE system. Aplurality of IMRs may be configured for one UE and each of the pluralIMRs may be independently configured. That is, the period, offset, andresource configuration of the IMR are independently determined and maybe signaled by an eNB to a UE using higher layer signaling (RRC etc.).

In addition, a CSI-RS is used to measure a channel desired for CoMP CSIcalculation in the LTE system. A plurality of CSI-RSs may be configuredfor one UE and each of the CSI-RSs in independently configured. Namely,each CSI-RS includes an independently configured period, offset,resource configuration, power control, and the number of antenna portsand information related to the CSI-RS is signaled to the UE from the eNBthrough higher layer signaling (RRC etc.).

Among a plurality of CSI-RSs and a plurality of IMRs configured for aUE, one CSI process may be defined in association with one CSI-RSresource for signal measurement and one IMR for interferencemeasurement. The UE feeds back CSI having different periods and subframeoffsets, derived from different CSI processes, to a network (e.g. eNB).

That is, each CSI process has an independent CSI feedback configuration.The eNB may signal the CSI-RS resource, IMR association information, andCSI feedback configuration to the UE through higher layer signaling ofRRC etc. on a CSI process basis. For example, it is assumed that threeCSI processes as shown in Table 1 are configured for the UE.

TABLE 1 Signal Measurement CSI Process Resource (SMR) IMR CSI process 0CSI-RS 0 IMR 0 CSI process 1 CSI-RS 1 IMR 1 CSI process 2 CSI-RS 0 IMR 2

In Table 1, CSI-RS 0 and CSI-RS 1 indicate a CSI-RS received from an eNB1 which is a serving eNB of the UE and a CSI-RS received from an eNB 2which is a neighboring eNB participating in cooperation. It is assumedthat IMRs configured for the CSI processes of Table 1 are configured asshown in Table 2.

TABLE 2 IMR eNB 1 eNB 2 IMR 0 Muting Data transmission IMR 1 Datatransmission Muting IMR 2 Muting Muting

In IMR 0, the eNB 1 performs muting, the eNB 2 performs datatransmission, and the UE is configured to measure interference of eNBsexcept for the eNB 1 from IMR 0. Similarly, in IMR 1, the eNB 2 performsmuting, the eNB 1 performs data transmission, and the UE is configuredto measure interference of eNBs except for the eNB 2 from IMR 1. Inaddition, in IMR 2, both the eNB 1 and eNB2 perform muting and the UE isconfigured to measure interference of eNBs except for the eNB 1 and eNB2 from IMR 2.

Accordingly, as shown in Table 1 and Table 2, CSI of CSI process 0indicates optimal RI, PMI, and CQI when data is received from the eNB 1.CSI of CSI process 1 indicates optimal RI, PMI, and CQI when data isreceived from the eNB 2. CSI of CSI process 2 indicates optimal RI, PMI,and CQI, when data is received from the eNB 1 and there is nointerference from the eNB 2.

Collision of CSI of CoMP

For CoMP scheduling, a UE should feed not only channel information of aserving cell or a serving transmission point (TP) but also channelinformation of a neighboring cell or TP participating in CoMP back to aBS. Accordingly, for CoMP, a UE feeds back CSI according to a pluralityof CSI processes considering an interference environment with aplurality of cells or TPs.

One CSI process is defined as association between one CSI-RS resourcefor signal measurement and one interference measurement resource (IMR)for interference measurement. In addition, each CSI process has anindependent CSI feedback configuration. The CSI feedback configurationincludes a feedback mode, a feedback period and an offset.

It is desirable that CSI processes configured for one UE share adependent value for CoMP scheduling efficiency. For example, if a firstcell and a second cell are subjected to joint transmission (JT), RIs andsubband indices of a first CSI process for the first cell and a secondCSI process for the second cell should be the same in order tofacilitate JT scheduling.

Accordingly, some or all of the CSI processes configured for the UE maybe restricted to have a common CSI (e.g., RI) value. For convenience ofdescription, among the CSI processes restricted to have the common CSIvalue, a CSI process which is used as a reference for setting a CSIvalue is referred to as a reference CSI process and the CSI processesother than the reference CSI process are referred to as following CSIprocesses. The following CSI process may feed the same value as the CSIvalue of the reference CSI process back without separate computation.

Here, since the CSI feedback configuration of each CSI process may beindependently set, collision between CSI processes may occur. Forexample, a reporting type of one CSI process and a reporting type ofanother CSI process may be configured to be fed back at the same timesuch that collision between the CSI processes may occur. Morespecifically, when CSI feedback is performed according to a plurality ofCSI processes each having a constant period and offset, the plurality ofCSI processes may be fed back on the same subframe, such that collisionmay occur.

Hereinafter, a method of handling collision between reporting typesincluding an RI if collision between CSI processes occurs is proposed.For example, this method is applicable to collision occurring among type3, type 5 and type 6 among CSI reporting types defined in LTE release10. A CSI reporting type defined in LTE release 10 will now bedescribed.

A type 1 report supports CQI feedback for a UE in a selected subband. Atype 1 a report supports subband CQI and second PMI feedback. Type 2,type 2b type 2c reports support broadband CQI and PMI feedback. A type2a report supports broadband PMI feedback. A type 3 report supports RIfeedback. A type 4 report supports broadband CQI. A type 5 reportsupports RI and broadband PMI feedback. A type 6 report supports RI andPTI feedback.

According to definition of LTE release 10, if collision between CSIprocesses occurs, drop priority is first determined according to areporting type. If drop priorities according to the reporting type arethe same, a CSI process having a lower CSI process index has higherpriority. CSI reporting types 3, 5 and 6 have the same priority. Sincethe priorities according to the reporting type are the same, a CSIprocess excluding a CSI process having a lowest index is dropped.

Hereinafter, a method of handling collision between a type 6 report of afollowing CSI process and a type 3, type 5 or type 6 report of areference CSI process will be described.

According to the present invention, a UE preferentially feeds a reportof a reference CSI process back and drops a type 6 report of a followingCSI process. That is, the index of the reference CSI process may be setto be lower than the index of the following CSI process. At this time,the type 6 report of the following CSI process is dropped together witha PTI joint encoded with an RI. The UE may determine the dropped PTIvalue using the following method.

First, the UE may determine the PTI value of the following CSI processto be the PTI value of the reference CSI process.

More specifically, if the type 6 report of the following CSI process andthe type 3, type 5 or type 6 report of the reference CSI processcollide, the UE determines the PTI value of the following CSI process tobe the PTI value of the currently fed-back reference CSI process. Thatis, after the collision time, the UE calculates and reports CQI or PMIof the following CSI process based on the PTI value of the reference CSIprocess. Thereafter, if the UE feeds the type 6 report of the followingCSI process back without collision, the UE calculates CQI or PMI basedon the PTI value of the newly fed-back following CSI process, not basedon the PTI value of the reference CQI process.

Next, the UE may determine the PTI value of the following CSI process tobe a default PTI value.

More specifically, if the type 6 report of the following CSI process andthe type 3, type 5 or type 6 report of the reference CSI processcollide, the UE may determine the PTI value of the following CSI processto be a default PTI value. The default PTI value may be 0 or 1 and theBS and the UE may share the predetermined default PTI value. Thereafter,if the UE feeds the type 6 report of the following CSI process backwithout collision, the UE calculates CQI or PMI based on the PTI valueof the newly fed-back following CSI process, not based on the defaultPTI value.

Next, the UE may determine the PTI value of the following CSI process tobe a recently reported PTI value according to the following CSI process.

More specifically, if the type 6 report of the following CSI process andthe type 3, type 5 or type 6 report of the reference CSI processcollide, the UE may determine the PTI value of the following CSI processto be a recently reported PTI value according to the following CSIprocess. Thereafter, if the UE feeds the type 6 report of the followingCSI process back without collision, the UE calculates CQI or PMI basedon the PTI value of the newly fed-back following CSI process, not basedon the recently reported PTI value, according to the following CSIprocess.

If the type 6 report of the following CSI process and the type 3, type 5or type 6 report of the reference CSI process collide, the UE maymultiplex the PTI value of the following CSI process with the referenceCSI process and report the multiplexed result.

Hereinafter, a method of handling collision between the type 5 report ofthe following CSI process and the type 3, type 5 or type 6 report of thereference CSI process will be described. That is, the case in which,instead of the type 6 report of the following CSI process of theabove-described method, the type 5 report of the following CSI processcollides with the type 3, type 5 or type 6 report of the reference CSIprocess will be described.

According to the present invention, the UE preferentially feeds a reportof a reference CSI process back and drops a type 5 report of a followingCSI process. That is, the index of the reference CSI process may be setto be lower than the index of the following CSI process. At this time,the type 5 report of the following CSI process is dropped together witha PMI (W1) joint encoded with an RI. The UE may determine the dropped W1value using the following method.

First, the UE may determine the W1 value of the following CSI process tobe the W1 value of the reference CSI process.

More specifically, if the type 5 report of the following CSI process andthe type 5 report of the reference CSI process collide, the UEdetermines the W1 value of the following CSI process to be the W1 valueof the currently fed-back reference CSI process. That is, after thecollision time, the UE calculates and reports CQI or PMI of thefollowing CSI process based on the W1 value of the reference CSIprocess. Thereafter, if the UE feeds the type 5 report of the followingCSI process back without collision, the UE calculates CQI or PMI basedon the W1 value of the newly fed-back following CSI process, not basedon the W1 value of the reference CQI process.

FIG. 17 shows an example of determining the W1 value of the followingCSI process to be the W1 value of the reference CSI process if the type5 report of the following CSI process and the type 5 report of thereference CSI process collide.

Referring to FIG. 17, if the type 5 reports of CSI process 1, which isthe reference CSI process, and CSI process 2, which is the following CSIprocess, collide, the UE drops the type 5 report of CSI process 2 whichis the following CSI process. After the type 5 report of CSI process 2is dropped, the UE calculates and reports CQI or PMI of CSI process 2,which is the following CSI process, based on the W1 value of CSI process1 which is the reference CSI process.

Next, the UE may determine the W1 value of the following CSI process tobe a default W1 value.

More specifically, if the type 5 report of the following CSI process andthe type 3, type 5 or type 6 report of the reference CSI processcollide, the UE may determine the W1 value of the following CSI processto be a default W1 value. The default W1 value may be 0 or 1 and the BSand the UE may share the predetermined default W1 value. Thereafter, ifthe UE feeds the type 5 report of the following CSI process back withoutcollision, the UE calculates CQI or PMI based on the W1 value of thenewly fed-back following CSI process, not based on the default W1 value.

Next, the UE may determine the W1 value of the following CSI process tobe a recently reported W1 value according to the following CSI process.

More specifically, if the type 5 report of the following CSI process andthe type 3, type 5 or type 6 report of the reference CSI processcollide, the UE may determine the W1 value of the following CSI processto be a recently reported W1 value according to the following CSIprocess. Thereafter, if the UE feeds the type 5 report of the followingCSI process back without collision, the UE calculates CQI or PMI basedon the W1 value of the newly fed-back following CSI process, not basedon the recently reported PTI value, according to the following CSIprocess.

If the type 5 report of the following CSI process and the type 3, type 5or type 6 report of the reference CSI process collide, the UE maymultiplex the W1 value of the following CSI process with the referenceCSI process and report the multiplexed result.

FIG. 18 is a diagram showing another embodiment of the case in which atype 5 report of a following CSI process and a type 5 report of areference CSI process collide.

If the type 5 report of the following CSI process and the type 5 reportof the reference CSI process collide, the UE may determine priorityaccording to the following drop rule without preferentially consideringthe report of the reference CSI process. Upon collision between CSIprocesses, the UE may assign higher priority in order of a reportingtype, a CSI process index and a component carrier (CC) index. At thistime, the situation shown in FIG. 18 may occur.

Referring to FIG. 18, the following CSI process has a CSI process indexof 1, the reference CSI process has a CSI process index of 2, and thetwo CSI process collide at a specific time. According to theabove-described drop rule, since the reporting types of the two CSIprocesses are the same, the UE determines priority according to the CSIprocess index. Accordingly, the UE drops the CSI of the reference CSIprocess having the higher CSI process index. At this time, the RI of thefollowing CSI process inherits the recently reported RI value accordingto the reference CSI process. The W1 value of the following CSI processsubjected to joint encoding may not be inherited but may beindependently determined. In case of FIG. 17, since the W1 value of thefollowing CSI process is also dropped, the W1 value of the reference CSIprocess is efficiently inherited. However, in case of FIG. 18, since theW1 value of the following CSI process is not dropped, the W1 value ofthe following CSI process may be independently determined. In FIG. 18,after collision, the W2 and CQI value of the following CSI process arecalculated based on the recently reported RI and W1 value. At this time,the RI is the RI value of the reference CSI process before collision andW1 may be independently determined based on the RI value in thefollowing CSI process.

FIG. 19 is a diagram showing an embodiment in which three CSI processescollide as an extension of FIG. 18.

Referring to FIG. 19, CSI processes 1 and 2 are configured as followingCSI processes, a CSI process 3 is configured as a reference CSI processand the three CSI processes collide at a specific time. According to theabove-described drop rule, the CSI process 2 having a higher CSI processindex and the CSI process 3 which is the reference CSI process aredropped. In this case, the RI of the CSI process 1 inherits a recentlyreported RI value according to the reference CSI process. The W1 valuesubjected to joint encoding may be independently determined instead ofbeing inherited. The CSI process 2 inherits the RI and W1 value of theCSI process 1. That is, if the reference CSI process and two or morefollowing CSI process collide, one following CSI process inherits thevalue of the remaining following CSI process if both the report thereofand the report of the reference process are dropped. In FIG. 19, the RIof the CSI process 2 inherits the RI value of the CSI process 1. The W1value of the CSI process 2 inherits the W1 value of the CSI process 1.The W1 value of the CSI process 1 is determined independently of thereference CSI process. Therefore, the CSI process 2 does not inherit theW1 value of the reference CSI process but inherits the value of theremaining following CSI process.

Although the RI and the PMI are subjected to joint encoding in FIG. 19,the following CSI process inheriting the value of the remainingfollowing CSI process if the reference CSI process and two or morefollowing CSI process collide is applicable to the case in which onlythe RI value is reported or the RI and the PTI are subjected to jointencoding.

As in the embodiment of FIG. 18 or 19, if the index of the reference CSIprocess is higher than the index of the following CSI process, thereference CSI process is dropped and the inherited RI value of thereference CSI process becomes a previous value. That is, since previouschannel information is reported, CSI feedback accuracy is decreased.Accordingly, when the reference CSI process and the following CSIprocess collide, the index of the reference CSI process is preferablyset to be lower than the index of the following CSI process such thatthe reference CSI process is not dropped. Alternatively, the index ofthe reference CSI process may be set to be fixed to 1 which is a lowestCSI process index. In this case, the UE anticipates that the BS sets theindex of the reference CSI process to 1.

If the index of the reference CSI process is higher than the index ofthe following CSI process and the periods and offsets of the two CSIprocesses are the same such that collision between the CSI processesalways occurs, the reference CSI process is always dropped and the valueinherited by the following CSI process may be removed. This problem maybe resolved using the following two methods. First, if the index of thereference CSI process is set to be higher than the index of thefollowing CSI process, the periods and offsets of the two CSI processesmay not be set to be equal. Next, if the periods and offsets of thereference CSI process and the following CSI process are the same, theindex of the reference CSI process is not set to be higher than theindex of the following CSI process. Alternatively, the index of thereference CSI process may be set to 1.

Contradiction in Common CSI Application in CoMP

Codebook subset restriction means that a UE is restricted to select aprecoder only within a subset including elements in a codebook. That is,codebook subset restriction means that a codebook including variousprecoding matrices is generated and then available precoding matricesare restricted according to the cell or UE. If codebook subsetrestriction is used, an overall wireless communication system has acodebook having a-large size but a codebook used by each UE includes asubset of a codebook. Thus, precoding gain may be increased.

Here, if codebook subset restriction is independently set according tothe CSI process, the RI of the following CSI process and the RI (commonRI) of the reference CSI process may not be set to the same value. Thatis, a problem may occur upon application of the common RI due tocodebook subset restrictions. For example, if codebook subsetrestriction is set such that the reference CSI process uses ranks 1 and2 and the following CSI process uses only rank 1, available RIs aredifferent and thus a problem may occur. That is, if the RI of thereference CSI process is 2, the rank of the following CSI process maynot be set to 2 due to codebook subset restriction. In this case, the UEmay perform the following procedure.

First, the UE may determine and feed the RI of the following CSI processback separately from the RI of the reference CSI process. In this case,codebook subset restriction is applied ahead of the RI of the referenceCSI process. Accordingly, in this case, the common RI is not applied.When the RI of the following CSI process is selected, the UE determinesavailable RIs according to the codebook subset restriction of thefollowing CSI process and selects an optimal RI from among the availableRIs based on a non zero power (NZP) CSI and IMR measurement value of thefollowing CSI process.

Next, the UE may determine the RI of the following CSI process to be thesame value as the RI of the reference CSI process. In this case, the RIof the reference CSI process is applied ahead of codebook subsetrestriction. Accordingly, in this case, the codebook subset restrictionof the following CSI process is not applied.

Next, available RIs may be determined according to codebook subsetrestriction of the following CSI process and an RI most approximate tothe RI of the reference CSI process may be selected from among theavailable RIs. In case of periodic feedback, the RI of the following CSIprocess means a most recent value among values reported when or beforethe RI of the following CSI process is reported. In case of aperiodicfeedback, the RI of the following CSI process means a value reported atthe same time as the RI of the following CSI process.

Next, available RIs may be determined according to codebook subsetrestriction of the following CSI process and a smallest RI may beselected from among the available RIs.

As described above, in order to prevent a contradiction between codebooksubset restriction of the following CSI process and application of thecommon RI, codebook subset restriction may not be independently setaccording to the CSI process. That is, the BS may set the same codebooksubset restriction with respect to the following CSI process and thereference CSI process and the UE may anticipate that the codebook subsetrestriction of the following CSI process is equal to the codebook subsetrestriction of the reference CSI process.

In addition, in order to prevent the above-described problems, the BSmay set the codebook subset restriction of the following CSI process andthe codebook subset restriction of the reference CSI process such thatthe available RI of the following CSI process is equal to the availableRI of the reference CSI process. Similarly, the UE may not anticipatethat the codebook subset restriction of the following CSI process andthe codebook subset restriction of the reference CSI process are setsuch that the available RI of the following CSI process and theavailable RI of the reference CSI process are different.

In order to prevent the above-described problem, the BS may set thecodebook subset restriction of the following CSI process and thecodebook subset restriction of the reference CSI process such that a setof available RIs of the following CSI process is equal to a set ofavailable RIs of the reference CSI process or a superset thereof. Thatis, the UE may anticipate that the codebook subset restriction of thefollowing CSI process and the codebook subset restriction of thereference CSI process are set such that a set of available RIs of thefollowing CSI process is equal to a set of available RIs of thereference CSI process or a superset thereof. Similarly, the UE may notanticipate that the codebook subset restriction of the following CSIprocess and the codebook subset restriction of the reference CSI processare set such that a set of available RIs of the following CSI process isnot included in a set of available RIs of the reference CSI process.

Although the above-described features describe the case in which a thereis a contradiction between the codebook subset restriction of thefollowing CSI process and the application of the common RI, the presentinvention is not limited thereto and is applicable to the case in whichthere is a contradiction between the application of the common PMI andthe codebook subset restriction of the following CSI process.Hereinafter, a procedure when there is a contradiction between theapplication of the common PMI and the codebook subset restriction of thefollowing CSI process will be described.

First, the UE may determine and feed the PMI of the following CSIprocess back separately from the PMI of the reference CSI process. Inthis case, codebook subset restriction is applied ahead of the PMI ofthe reference CSI process. Accordingly, in this case, the common PMI isnot applied. When the PMI of the following CSI process is selected, theUE determines available PMIs according to codebook subset restriction ofthe following CSI process and selects an optimal RI from among theavailable PMIs based on a non zero power (NZP) CSI and IMR measurementvalue of the following CSI process.

Next, the UE may determine the PMI of the following CSI process to bethe same value as the PMI of the reference CSI process. In this case,the PMI of the reference CSI process is applied ahead of codebook subsetrestriction. Accordingly, in this case, the codebook subset restrictionof the following CSI process is not applied.

Next, available PMIs may be determined according to the codebook subsetrestriction of the following CSI process and a PMI most approximate tothe PMI of the reference CSI process may be selected from among theavailable PMIs. For example, an approximation degree between two PMIsmay be determined based on Euclidean distance or correlation between thetwo PMIs. More specifically, as correlation is increased or as Euclideandistance is decreased, it may be determined that the two PMIs areapproximate to each other. In case of periodic feedback, the PMI of thefollowing CSI process means a most recent value among values reportedwhen or before the PMI of the following CSI process is reported. In caseof aperiodic feedback, the PMI of the following CSI process means avalue reported at the same time as the PMI of the following CSI process.

Next, available PMIs may be determined according to the codebook subsetrestriction of the following CSI process and a smallest PMI may beselected from among the available PMIs.

As described above, in order to prevent a contradiction between codebooksubset restriction of the following CSI process and application of thecommon CSI, codebook subset restriction may not be independently setaccording to the CSI process. That is, the BS may set the same codebooksubset restriction with respect to the following CSI process and thereference CSI process and the UE may anticipate that the codebook subsetrestriction of the following CSI process is equal to the codebook subsetrestriction of the reference CSI process.

Hereinafter, similarly to a contradiction between codebook subsetrestriction and common CSI, the case in which the number of CSI-RSantenna ports of the following CSI process and the number of CSI-RSantenna ports of the reference CSI process are different will bedescribed.

If the number of CSI-RS antenna ports of the following CSI process andthe number of CSI-RS antenna ports of the reference CSI process aredifferent, it is impossible to equally set the RIs and PMIs of the twoCSI processes. For example, if the number of CSI-RS antenna ports of thefollowing CSI process and the number of CSI-RS antenna ports of thereference CSI process are respectively set to 4 and 8 and the RI of thereference CSI process is set to 8, the RI of the following CSI processmay not be equally set to 8.

In order to solve such a problem, the BS may equally set the number ofCSI-RS antenna ports of the following CSI process and the number ofCSI-RS antenna ports of the reference CSI process. At this time, the UEmay anticipate that the number of CSI-RS antenna ports of the followingCSI process and the number of CSI-RS antenna ports of the reference CSIprocess are the same. Similarly, the UE may not anticipate that thenumber of CSI-RS antenna ports of the following CSI process and thenumber of CSI-RS antenna ports of the reference CSI process aredifferent.

As another embodiment, the BS may set the number of CSI-RS antenna portsof the following CSI process to be equal to or greater than the numberof CSI-RS antenna ports of the reference CSI process. That is, the UEmay anticipate that the number of CSI-RS antenna ports of the followingCSI process is equal to or greater than the number of CSI-RS antennaports of the reference CSI process. If the number of CSI-RS antennaports of the following CSI process is equal to or greater than thenumber of CSI-RS antenna ports of the reference CSI process, no problemoccurs.

As another method, if the number of CSI-RS antenna ports of thefollowing CSI process and the number of CSI-RS antenna ports of thereference CSI process are different, the UE may calculate the RI and PMIof the following CSI process separately from the RI and PMI of thereference CSI process. Alternatively, if the number of CSI-RS antennaports of the following CSI process is less than the number of CSI-RSantenna ports of the reference CSI process, the UE may calculate the RIand PMI of the following CSI process separately from the RI and PMI ofthe reference CSI process.

Hereinafter, a contradiction in the application of the common CSI, whichoccurs if a configuration for activation of the RI and PMI report isindependent in each CSI process, will be described.

If the configuration for activation of the RI and PMI report isindependent according to the CSI process, the RI of the following CSIprocess may not be determined to be the same value as the RI of thereference CSI process. For example, although the RI and PMI report ofthe reference CSI process is activated and the RI is set to 2, if the RIand PMI report of the following CSI process is deactivated, the rank ofthe following CSI process may not be set to 2. In this case, the UE mayperform the following procedure.

First, the RI and PMI report of the following CSI process may bedeactivated. In this case, a configuration for deactivation of the RIreport of the following CSI process is applied ahead of application ofthe RI of the reference CSI process. At this time, the RI of thereference CSI process is not applied.

Next, the RI of the following CSI process may be determined to be thesame value as the RI of the reference CSI process. In this case,application of the RI of the reference CSI process is applied ahead ofthe configuration for deactivation of the RI and PMI report of thefollowing CSI process. At this time, the configuration for deactivationof the RI and PMI report of the following CSI process is not valid.

In order to prevent the above-described problem, the RI and PMI reportsof the following CSI process and the reference CSI process may remainconstantly activated. At this time, the BS may configure both the RI andPMI reports of the following CSI process and the reference CSI processto be activated. The UE may anticipate that both the RI and PMI reportsof the following CSI process and the reference CSI process areactivated.

Priority Upon Collision Between CSI Processes

Hereinafter, a method of determining reported CSI and dropped CSIaccording to priority if two or more CSI processes collide in periodicCSI feedback using a PUCCH will be described.

Upon collision between CSI processes, priority of CSI reportingcurrently defined in LTE release 10 is as follows. Upon collisionbetween CSI processes, the UE assigns higher priority in order of areporting type, a CSI process index and a CC index.

For example, priority of the reporting type is first considered andthen, if the priorities of the reporting types are the same, a lowerindex has higher priority based on the CSI process index. If thepriorities of the reporting types are the same and the CSI processindices are the same, a CSI process having a lower CC index has higherpriority.

The priority according to the reporting type is determined as follows.In a corresponding subframe, if the CSI report of PUCCH reporting type3, 5, 6 or 2a collides with the CSI report of PUCCH reporting type 1,1a, 2, 2b, 2c or 4, the latter CSI report has low priority and isdropped. In a corresponding subframe, if the CSI report of PUCCHreporting type 2, 2b, 2c or 4 collides with the CSI report of the PUCCHreporting type 1 or 1a, the latter CSI report has low priority and isdropped.

In the present invention, details of the priority of the conventionalreporting type are proposed. According to the present invention, in acorresponding subframe, if the CSI report of PUCCH reporting type 5 or 6collides with the CSI report of the PUCCH reporting type 3, the latterCSI report has low priority and is dropped.

The priorities of the above-described PUCCH reporting type 3, 5 and 6may be applied upon collision between the reference CSI process and thefollowing CSI process. For example, if the reporting type 6 of thefollowing CSI process and the reporting type 3 of the reference CSIprocess collide in the same subframe, the CSI report of the reportingtype 3 is dropped and the CSI of the reporting type 6 of the followingCSI process is reported.

Since not only the RI but also the PTI are subjected to joint encodingin the PUCCH reporting type 6, not only the RI but also the PTI valuemay be reported without loss, by applying the priority of the presentinvention. Similarly, since not only the RI but also the W1 value aresubjected to joint encoding in the PUCCH reporting type 5, not only theRI but also the W1 value may be reported without loss, by applying theabove-described priority.

At this time, the RI value of the reference CSI process is dropped butthe same RI value as the RI of the reference process is reported viatype 5 or 6. Therefore, the UE calculates the PMI and CQI of thereference CSI process based on the RI value of type 5 or 6 until the RIof a next reference CSI process is reported.

In a conventional system, an ACK/NACK report for data and CSI(RI/PMI/subband index) feedback collide, the ACK/NACK report ispreferentially considered and the CSI is dropped. However, if the CSI ofthe reference CSI and the ACK/NACK report collide, the CSI report of thereference CSI process preferably has priority higher than that of theACK/NACK report. The CSI of the reference SI process is reported and theACK/NACK report is dropped. Since the CSI of the reference CSI processis referred to by one or more following CSI processes, if the CSI reportof the reference CSI process is dropped, the CSI value of the followingCSI process may be influenced. Accordingly, if the CSI of the referenceCSI process and the ACK/NACK report collide, the CSI report of thereference CSI process preferably has priority higher than that of theACK/NACK report.

FIG. 20 is a diagram showing a BS and a UE which are applicable to thepresent invention.

If a wireless communication system includes a relay, communication in abackhaul link is performed between the BS and the relay andcommunication in an access link is performed between the relay and theUE. Accordingly, the BS and UE shown in FIG. 20 may be replaced with therelay according to situation.

Referring to FIG. 20, a wireless communication system includes a BS 110and a UE 120. The BS 110 includes a processor 112, a memory 114, and aRadio Frequency (RF) unit 116. The processor 112 may be configured so asto implement the procedures and/or methods proposed in the presentinvention. The memory 114 is connected to the processor 112 and storesvarious pieces of information related to operations of the processor112. The RF unit 116 is connected to the processor 112 and transmitsand/or receives RF signals. The UE 120 includes a processor 122, amemory 124, and an RF unit 126. The processor 122 may be configured soas to implement the procedures and/or methods proposed in the presentinvention. The memory 124 is connected to the processor 122 and storesvarious pieces of information related to operations of the processor122. The RF unit 126 is connected to the processor 122 and transmitsand/or receives RF signals. The BS 110 and/or the UE 120 may have asingle antenna or multiple antennas.

The embodiments of the present invention described hereinabove arecombinations of elements and features of the present invention. Theelements or features may be considered selective unless otherwisementioned. Each element or feature may be practiced without beingcombined with other elements or features. Further, an embodiment of thepresent invention may be constructed by combining parts of the elementsand/or features. Operation orders described in the embodiments of thepresent invention may be rearranged. Some constructions of any oneembodiment may be included in another embodiment and may be replacedwith corresponding constructions of another embodiment. It is obviousthat claims that are not explicitly cited in each other in the appendedclaims may be presented in combination as an embodiment of the presentinvention or included as a new claim by subsequent amendment after theapplication is filed.

In this document, the embodiments of the present invention have beendescribed centering on a data transmission and reception relationshipbetween a UE and a BS. In some cases, a specific operation described asperformed by the BS may be performed by an upper node of the BS. Namely,it is apparent that, in a network comprised of a plurality of networknodes including a BS, various operations performed for communicationwith a UE may be performed by the BS, or network nodes other than theBS. The term BS may be replaced with the terms fixed station, Node B,eNode B (eNB), access point, etc.

The embodiments of the present invention may be achieved by variousmeans, for example, hardware, firmware, software, or a combinationthereof. In a hardware configuration, the embodiments of the presentinvention may be achieved by one or more Application Specific IntegratedCircuits (ASICs), Digital Signal Processors (DSPs), Digital SignalProcessing Devices (DSPDs), Programmable Logic Devices (PLDs), FieldProgrammable Gate Arrays (FPGAs), processors, controllers,microcontrollers, microprocessors, etc.

In a firmware or software configuration, the embodiments of the presentinvention may be implemented in the form of a module, a procedure, afunction, etc. For example, software code may be stored in a memory unitand executed by a processor.

The memory unit is located at the interior or exterior of the processorand may transmit and receive data to and from the processor via variousknown means.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of theinvention should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein.

MODE FOR INVENTION

Various embodiments have been described in the best mode for carryingout the invention.

INDUSTRIAL APPLICABILITY

The present invention is applicable to a wireless communicationapparatus such as a UE, a relay or a BS.

The invention claimed is:
 1. A method for receiving a CSI-RS (ChannelState Information-Resource Signal) by a user equipment (UE) in awireless access system, the method comprising: receiving a configurationfor a CSI (Channel State Information) process; receiving the CSI-RSassociated with the CSI process from a base station, wherein, when a RI(Rank Indicator)-reference CSI process is configured, the UE is notexpected to receive the configuration for the CSI process andconfiguration for the RI-reference CSI process that have a differentnumber of CSI-RS antenna ports.
 2. The method of claim 1, wherein a RIof the CSI process is configured the same as a RI of the RI-referenceCSI process.
 3. The method of claim 1, wherein the configurations forthe CSI process and the RI-reference CSI process are received throughRRC (Radio Resource Control) layer signaling.
 4. The method of claim 1,wherein the CSI includes at least one of a RI, a PMI (Precoding MatrixIndicator), and a CQI (Channel Quality Indicator).
 5. A user equipment(UE) in a wireless access system, the UE comprising: a RF (RadioFrequency) module; and a processor that controls the RF module to:receive a configuration for a CSI (Channel State Information) process;receive a CSI-RS (Channel State Information-Resource Signal) associatedwith the CSI process from a base station, wherein, when a RI (RankIndicator)-reference CSI process is configured, the UE is not expectedto receive the configuration for the CSI process and configuration forthe RI-reference CSI process that have a different number of CSI-RSantenna ports.
 6. The UE of claim 5, wherein a RI of the CSI process isconfigured the same as a RI of the RI-reference CSI process.
 7. The UEof claim 5, wherein the configurations for the CSI process and theRI-reference CSI process are received through RRC (Radio ResourceControl) layer signaling.
 8. The UE of claim 5, wherein the CSI includesat least one of a RI, a PMI (Precoding Matrix Indicator), and a CQI(Channel Quality Indicator).